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INTRODUCTORY  ESSAY.                                   83
earth) is usually estimated at ,one degree for three hundred
feet. In India, it is only in the most perennially humid and
densely wooded mountains, that the diminution of tempera-
ture is so rapid as this, for in the drier districts it is very
much less. Thus, while in Sikkim 1 for 300 feet is the pro-
portion for elevations below 7000 feet, on the Nilghiri Hills it
is about 1 for 340 feet, in Khasia 1 for 380 feet; and the
elevations of Nagpur and Ambala produce no perceptible di-
minution in their mean 'temperature, jrhich is as great as
that which would normally be assigned to them were they at
the level of the sea.
When the latitude, the amount of land, the humidity, and
the elevation are known, we have every element which influ-
ences climate; and as the limits between which each of these
elements varies is in India considerable, it is evident that
the diversity in the climate of its parts must be very great.
We reserve the details of these to the following chapter, and
shall confine ourselves here to pointing out the two broad
divisions^ of climates, which it is important to bear in mind,
namely, those which are excessive, and those which are equable.
An equable climate prevails in the vicinity of the equator,
and in all perennially humid districts; while an excessive cli-
mate, in which the summer is very hot and the winter cold,
is characteristic of the north-western regions, of the interior
of the continent, and of provinces characterized by extreme
drought. The northern districts of India are more exces-
sive in climate than the southern, because they ar<j broader
expanses of land; and the western side of the great (Madras)
peninsula is more equable than the eastern, because it is much
more humid.
VI. Sketch of the Physical Features and Vegetation of the
Provinces of India.
A. Limits of the ' Flora Indica.'
Although the main object of this Flora is the illustration